Communications apparatus and method

ABSTRACT

A method of reusing frequency bands between base stations of a terrestrial mobile communications network and a satellite communications network, comprising allocating the frequency bands using integrated resource management and other mitigation techniques in a such a way as to minimize interference between both the systems, thus making optimum usage of valuable frequency spectrum.

FIELD OF THE INVENTION

[0001] This invention relates to communications with a mobile user, andparticularly to such communications in which links to mobile users arevia a satellite or satellites, and also a relay station of a terrestrialnetwork.

BACKGROUND OF THE INVENTION

[0002] Mobile satellite communication systems (MSSs) providing globalcoverage are known. One such is the Iridium™ system, others are the ICO™system the Globalstar System™, and the Teledesic™ system.

[0003] Since such systems operate globally (or at least, over a largepart of the Earth's surface) they need to use a band of frequencieswhich are available all round the Earth.

[0004] Such MSS systems have inherent limitations in their capability toprovide services to users who are indoors and are in dense urban areas.Thus the available frequencies for these systems are wasted in denseurban areas and indoors.

[0005] Various Terrestrial mobile communications providing localgeographic coverage are know. Known systems include GSM and itsvariants, CDMA, IS-136 and a variety of others using time divisionmultiple access (TDMA) and code division multiple access (CDMA)techniques.

[0006] Code division multiple access is a so-called “spread spectrum”system, in which a given mobile device communicates using a relativelywide band, produced by multiplying the digital signal with a high bitrate (“chip rate”) code sequence. Each code sequence defines a separatecode channel.

[0007] Such systems, even though they are efficient and cost-effectivein providing high capacity and coverage indoors and in dense urbanareas, are not efficient and cost effective in terms of providingcoverage to vast thinly populated rural areas.

[0008] Ideally, the satellite and terrestrial communications systemscould be allocated completely separately frequency ranges, and theywould then not interfere with each other.

[0009] Known systems like Iridium™ and Globalstar™ rely on roamingbetween satellite and terrestrial systems, and use completely differentfrequency spectrum for accessing the satellite and terrestrial systems.However, roaming between satellite and terrestrial systems would be awaste of valuable spectrum, considering that the spectrum used forSatellite communication system cannot be used in dense urban areas andindoors, while spectrum allocated for terrestrial use is not deployed inrural and ocean areas.

[0010] Accordingly, the present invention is designed to increase thepossibilities for reusing the same channels (for example frequencychannels) between terrestrial and satellite mobile communicationsystems.

[0011] U.S. Pat. No. 5,394,561 discloses a mechanism for networkingsatellite and terrestrial networks in which the power levels of thesatellite and terrestrial communications are controlled so as tominimise co-channel interference.

SUMMARY OF THE INVENTION

[0012] In one aspect, the invention provides a communications systemcomprising a satellite mobile communications network which comprises aplurality of satellites and a plurality of user terminals communicatingon satellite uplink and downlink bands; and a terrestrial mobilecommunications network which comprises a plurality of base stations anda plurality of user terminal communicating on terrestrial uplink anddownlink bands; characterised in that at least one of the terrestrialbands at least partly reuses at least one of the satellite bands.

[0013] Preferably, an embodiment provides a communication system wherethe resource management, allocation and planning functions of thesatellite and terrestrial systems are linked together in such a way asto allow planned reuse of the spectrum.

[0014] In another aspect, the present invention provides a frequencyreuse system comprising means for reducing localised reuse of saidsatellite uplink and/or downlink in regions around one of said basestations.

[0015] Where the satellite downlink (typically the mobile link) sharesthe same frequencies as the land network, dual mode terminals will beable to use the terrestrial network instead of the satellite network,and interference from the satellite downlink into the land network isreduced. Indirectly, interference from the satellite uplink is alsoreduced since terminals cease to use the satellite service in theabsence of downlink.

[0016] In another aspect, the present invention provides a frequencyreuse system, comprising means for transmitting a control signal tosatellite user terminals in regions around one of said base stations tocause said user terminals to reduce (if necessary, to zero) use of saidsatellite uplink.

[0017] In another embodiment, the present invention provides a frequencyreuse system, comprising means for transmitting a control signal tosatellite user terminals in regions around one of said base stations tocause said user terminals to use channels which are non-interfering withsaid terrestrial network.

[0018] The control signal may be transmitted by the satellite. It may bea modified version of a predetermined common control signal.

[0019] In another embodiment, the control signal may be transmitted bythe land network. As in the preceding embodiment, where the satelliteuplink shares channels with the terrestrial network, interference fromthe satellite uplink is mitigated. Additionally, in this case, since theuser terminal responds to a signal transmitted by the terrestrialnetwork itself, rather than by the satellite as in the previous aspect,use of the shared spectrum on the satellite uplink is only suppressedwhen the user terminal is actually within range of the terrestrialnetwork.

[0020] In another aspect, the invention provides a dual mode userterminal in which the satellite system shares frequencies with theterrestrial system, and in which the user terminal is arranged to detectdownlink or uplink transmission on the terrestrial network, and to ceaseuse of the shared part of the satellite spectrum on detection thereof.Again, the terminal may cease transmission on the satellite system, butalternatively it may be switched to a non- interfering satellitechannel.

[0021] In one particular aspect, the uplink and downlink frequencies ofa terrestrial network reuse the same frequencies as the satellitedownlink but not the satellite uplink. This has the substantialadvantage that no uplink or downlink transmissions from the terrestrialnetwork are received by the satellite; such transmissions from a basestation or a large number of terrestrial handsets could be more powerfulthan the weak signals transmitted by a satellite handset and hence couldpotentially cause significant interference.

[0022] In the reverse direction, the satellite downlink is low powerbecause: firstly, the battery and solar cell power available on thesatellite is limited; secondly, the path length travelled is long; andthirdly, satellite terminals typically have higher sensitivity. Thus,the total power in the satellite downlink is low and causes minimalinterference.

[0023] In a particular preferred embodiment, this aspect of theinvention is employed with a satellite using narrowband frequency, orfrequency and time, division multiplexing and a terrestrial networkemploying CDMA. Where only a small number of satellite downlinktransmissions are taking place, the effect of these on each CDMA signalis limited since they occupy only a small part of the CDMA spectrum. Theinterference from the satellite is thus even less intrusive in thisembodiment.

[0024] This aspect is particularly preferably employed with the firstaspect of the invention, in which case because the terrestrial networkuses the satellite frequencies in shadowed areas (such as urban areasand indoors), the satellite downlink effect is reduced still furthersince the satellite signal is frequently shadowed.

[0025] In another aspect, the invention provides a satellite systemwhich reuses radio spectrum with a terrestrial communications network,in which the satellite uplink shares spectrum with the terrestrialuplink and the satellite downlink shares spectrum with the terrestrialdownlink. In this case, and particularly when this aspect is combinedwith the first, the satellite uplink causes relatively littleinterference at the terrestrial handset (and particularly when theterrestrial network uses spread spectrum communication and the satelliteuses narrowband frequency division or frequency and time divisionmultiplexing).

[0026] Also, in this embodiment or others it is particularly convenientto provide a dual mode user terminal having common elements of the radiofrequency transmit and receive circuit, to which a separate terrestrial(for example CDMA) and satellite (for example FDMA/TDMA) decoder anddemodulator are coupled.

[0027] In another aspect, the invention provides a satellite systemwhich reuses radio spectrum with a terrestrial communications network,in which the uplink and downlink frequencies of the terrestrial networkreuse the same frequencies as the satellite uplink but not the satellitedownlink.

[0028] This is particularly advantageous where data terminal equipmentis connected to the mobile terminals, since it is found that typical useof such data terminal equipment is heavily asymmetrical; that is, muchmore information is downloaded on the downlink (for example as a resultof downloading emails, and browsing or downloading files from theInternet) than is transmitted on the uplink (which typically carriesonly selection and navigation commands). There is therefore sparecapacity on the satellite uplink which can be reused for terrestrialcommunications.

[0029] Other aspects and preferred embodiments of the invention are asdescribed or claimed hereafter, with advantages which will be apparentfrom the following.

BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Embodiments of the invention will now be described, by way ofexample only, with reference to the accompanying drawings, in which:

[0031]FIG. 1 is a block diagram showing schematically the elements of afirst communications system embodying the present invention;

[0032]FIG. 2a is an illustrative diagram showing schematically theelements of mobile terminal equipment suitable for use with the presentinvention; and

[0033]FIG. 2b is a corresponding block diagram;

[0034]FIG. 3 is a block diagram showing schematically the elements of anEarth station node forming part of the embodiment of FIG. 1;

[0035]FIG. 4a illustrates schematically the beams produced by asatellite in the embodiment of FIG. 1;

[0036]FIG. 4b illustrates schematically the disposition of satellitesforming part of FIG. 1 in orbits around the earth;

[0037]FIG. 5 shows the arrangement of terrestrial base stations in thefirst embodiment;

[0038]FIG. 6 shows the frequency allocation in the first embodiment;

[0039]FIG. 7 shows the frequency allocation in the second embodiment;

[0040]FIG. 8 is a block diagram showing the user terminal of the secondembodiment;

[0041]FIG. 9 shows the frequency allocation in the third embodiment;

[0042]FIG. 10 shows the frequency allocation in the fourth embodiment;and

[0043]FIG. 11 illustrates the uplinks and downlinks present in theinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIRST EMBODIMENT

[0044] Referring to FIG. 1, a satellite communications network accordingto this embodiment comprises satellite mobile user terminal equipment 2a, 2 b (e.g. handsets 2 a and 2 b); orbiting relay satellites 4 a, 4 b;satellite earth station nodes 6 a, 6 b; satellite system gatewaystations 8 a, 8 b; terrestrial (e.g. public switched) telecommunicationsnetworks 10; and fixed telecommunications terminal equipment 12;terrestrial (e.g. public land) mobile telecommunications networks(PLMNs) 110 and terrestrial mobile terminal equipment 112.

[0045] Interconnecting the satellite system gateways 8 a, 8 b with theearth station nodes 6 a, 6 b, and interconnecting the nodes 6 a, 6 bwith each other, is a dedicated ground-based network comprising channels14 a, 14 b, 14 c. The satellites 4, earth station nodes 6 and lines 14make up the infrastructure of the satellite communications network, forcommunication with the mobile terminals 2, and accessible through thegateway stations 8.

[0046] A central database station 15 is connected, via a signalling link60 (e.g. within the channels 14 of the dedicated network) to the gatewaystation, earth stations 6, and PLMN 110 (as discussed below).

[0047] The PSTNs 10 comprise, typically, local exchanges 16 . . . towhich the fixed terminal equipment 12 . . . is connected via local loops18; and international switching centres 20 . . . connectable one toanother via national and transnational links 21 (for example, satellitelinks or subsea optical fibre cable links). The PSTNs 10 and fixedterminal equipment 12 (e.g. telephone instruments) are well known andalmost universally available today.

[0048] The PLMNs 110 comprise, typically, mobile switching centres(MSCs) 116 to which the terrestrial mobile terminals 112 are connectedvia local radio paths 118 and base stations 119; and internationalgateways 20 b.

[0049] In this embodiment, most or all of the satellite user terminals 2are dual mode and hence are also connectable via the terrestrial basestations 119.

[0050] For voice communications via the satellite network, eachsatellite mobile terminal apparatus 2 is in communication with asatellite 4 via a full duplex channel (in this embodiment) comprising adownlink (to-mobile) channel and an uplink (from-mobile) channel, forexample (in each case) a TDMA time slot on a particular frequencyallocated on initiation of a call, as disclosed in UK patentapplications GB 2288913 and GB 2293725. The satellites 4 in thisembodiment are non geostationary, and thus, periodically, there ishandover of the user terminal from one satellite 4 to another.

[0051] For voice communications via the PLMN, each mobile terminalapparatus 2, 112 is in communication with a mobile switching centre 116via a base station 119 using an uplink frequency band and a downlinkfrequency band.

[0052] Mobile Terminal 2

[0053] Referring to FIGS. 2a and 2 b, a dual mode mobile terminalhandset equipment 2 a of FIG. 1 is shown.

[0054] It comprises a combination of a satellite handset, similar tothose presently available for use with the GSM system, and a terrestrialhandset suitable for third generation (3G) CDMA, W-CDMA or CDMA 2000.communications.

[0055] The user interface components (microphone 36, loudspeaker 34,display 39 (for example a liquid crystal display) and keypad components38) and power supply (battery 40) are shared, i.e. used in both modes.

[0056] Apart from such common components (omitted for clarity from FIG.2b), the terminal comprises a CDMA functional unit 200 a and a satellitefunctional unit 200 b. Each comprises a digital coder/decoder 30 a, 30b; modem 32 a, 32 b; control circuit 37 a, 37 b; radio frequency (RF)interface 32 a and 32 b, and antennas 31 a and 31 b, suitable forsatellite and terrestrial mobile communications respectively. Thesatellite antenna 31 a has some gain in directions above the horizon (itmay be a Quadrifilar Helix or QFH antenna). The terrestrial antenna 31 bis roughly omnidirectional.

[0057] A ‘smart card’ reader 33 receiving a smart card (subscriberidentity module or SIM) 35 storing user information are also provided,connected to communicate with the satellite control circuit 37 b.Specifically, the SIM 35 includes a processor 35 a and permanent memory35 b.

[0058] The control circuits 37 a, 37 b (in practice integrated with therespective codec 30) consist of a suitably programmed microprocessor,microcontroller or digital signal processor (DSP) chip. Each controlcircuit 37 performs various functions including framing speech and datainto TDMA time frames for transmission (and likewise demultiplexingreceived TDMA frames) or CDMA sequences respectively; and performingencryption or enciphering.

[0059] Separate chipsets may be provided, each for implementing one ofthe terrestrial and satellite system functionalities 200 a, 200 b.Alternatively, a single processor may be programmed to perform thecoding and control for both functionalities. In each case, in thisembodiment separate RF components are provided, but user interfacecomponents are shared.

[0060] The mobile phone will then operate either as a satellitetelephone or as terrestrial phone, with the relevant functional unit 200a or 200 b working substantially independently and as it would do in asingle mode phone.

[0061] The coder/decoder (codec) 30, 30 b in this embodiment comprise acoder, generating a speech bit stream at around 3.6 kilobits per second,together with a channel coder 30 b applying error correcting encoding,to generate an encoded bit stream, and corresponding decoders.

[0062] In this embodiment the modems can support data rates of up to 384kbps also.

[0063] A single mode satellite handset 2 would be as described, butlacking the section 200 b; and a terrestrial handset 112 is as describedbut lacking section 200 a.

[0064] Earth Station Node 6

[0065] The earth station nodes 6 are arranged for communication with thesatellites.

[0066] The Earth stations 6 are positioned dispersed about the Earthsuch that for any orbital position, at least one Earth station 6 is inview of a satellite 4.

[0067] Each earth station node 6 comprises, as shown in FIG. 3, aconventional satellite earth station 22 consisting of at least onesatellite tracking antenna 24 arranged to track at least one satellite4, RF power amplifiers 26 a for supplying a signal to the antenna 24,and 26 b for receiving a signal from the antenna 24; and a control unit28 for storing the satellite ephemera data, controlling the steering ofthe antenna 24, and effecting any control of the satellite 4 that may berequired (by signalling via the antenna 24 to the satellite 4).

[0068] The earth station node 6 further comprises a mobile satelliteswitching centre 42 comprising a network switch 44 connected to thetrunk links 14 forming part of the dedicated network. A multiplexer 46is arranged to receive switched calls from the switch 44 and multiplexthem into a composite signal for supply to the amplifier 26 via a lowbit-rate voice codec 50. Finally, the earth station node 6 comprises alocal store 48 storing details of each mobile terminal equipment 2 awithin the area served by the satellite 4 with which the node 6 is incommunication.

[0069] Gateway 8

[0070] The gateway stations 8 a, 8 b comprise, in this embodiment,commercially available mobile switch centres (MSCs) of the type used indigital mobile cellular radio systems such as GSM systems.

[0071] The gateway stations 8 comprise a switch arranged to interconnectincoming PSTN lines from the PSTN 10 with dedicated service lines 14connected to one or more Earth station nodes 6.

[0072] Database Station 15

[0073] The database station 15 comprises a digital data store, asignalling circuit, a processor interconnected with the signallingcircuit and the store, and a signalling link 60 interconnecting thedatabase station 15 with the gateway stations 8 and Earth stations 6making up satellite system network, for signalling or data messagecommunications.

[0074] It stores data for terminal apparatus 2, for example positiondata, billing data, authentication data and so on, like the HomeLocation Register (HLR) of a GSM system.

[0075] Thus, in this embodiment the database station 15 acts to fulfilthe functions of a home location register (HLR) of a GSM system, and maybe based on commercially available GSM products.

[0076] Periodically, the Earth station nodes measure the delay andDoppler shift of communications from the terminals 2 and calculate therough terrestrial position of the mobile terminal apparatus 2 using thedifferential arrival times and/or Doppler shifts in the received signal.The position is then stored in the database 48.

[0077] The database station 15 in this embodiment also performsfrequency planning, to determine the frequencies to be used forcommunicating via the satellites 4 with each of the satellite userterminals 2, and to control the use of uplink and downlink frequenciesthereby as further discussed below. The database station 15 isaccordingly connected additionally with the radio spectrum allocationcomponents of the PLMN 110 via the signalling link 60; it can therebycommunicate with the Mobile Switching Centre (MSC) or Base StationControl centre which controls the frequencies used by the base stations119 b of the PLMN.

[0078] Periodically, the database station 15 transmits frequencyallocation information to the Earth stations 6 for use in the satellites4, and PLMNs 110.

[0079] Satellites 4

[0080] The satellites 4 a, 4 b comprise generally conventionalcommunications satellite buses such as the HS601 available from HughesAerospace Corp, California, US, and the payload may be as disclosed inGB 2288913. Each satellite 4 is arranged to generate an array of beamscovering a footprint beneath the satellite, each beam including a numberof different frequency channels and time slots, as described in GB2293725 and illustrated in FIG. 4a.

[0081] On each beam, the satellite therefore transmits a set of downlinkfrequencies. The downlink frequencies on adjacent beams are different,so as to permit frequency re-use between beams. Each beam therefore actssomewhat in the manner of a cell of a conventional terrestrial cellularsystem. For example, there may be 61, 121 or 163 beams.

[0082] In this embodiment each downlink frequency carries a plurality oftime division channels, so that each mobile terminal 2 communicates on achannel comprising a given time slot in a given frequency.

[0083] The satellites 4 a are arranged in a constellation in sufficientnumbers and suitable orbits to cover a substantial area of the globe(preferably to give global coverage).

[0084] Referring to FIG. 4b, a global coverage constellation ofsatellites is provided, consisting of a pair of orbital planes eachinclined at 45 degrees to the equatorial plane, spaced apart by 90degrees around the equatorial plane, each comprising ten pairs ofsatellites 4 a, 4 b, (i.e. a total of 20 operational satellites) thepairs being evenly spaced in orbit, with a phase interval of zerodegrees between the planes (i.e. a 10/2/0 constellation in Walkernotation) at an altitude of about 10,500 km (6 hour orbits).

[0085] Thus, neglecting blockages, a UT 2 at any position on Earth canalways have a communications path to at least one satellite 4 in orbit(“global coverage”).

[0086] Base Station 119

[0087] The base station 119 comprises a CDMA base station havingtransmit and receive antennas which are arranged to transmit signals ondownlink CDMA channels to mobile terminals, and to receive signals frommobile terminals on uplink CDMA channels. The downlink channels areprovided, in this embodiment, in a downlink frequency band and theuplink signals in an uplink frequency band. At the base station 119,there is further provided a conventional demodulator for demodulatingthe uplink signals to provide digital data and for modulating digitaldata onto the downlink signals. Each code channel may spread across theentire uplink or downlink spectrum in known fashion.

[0088] Referring to FIG. 5, the base stations 119 of this embodimentcomprise first base stations 119 a, each of which define a receptioncell around it, which are deployed in suburban and rural areas as wellas an in urban areas. In such cases, the effective radio coverage of thecell will be of the order of several kilometers or even tens ofkilometers, depending upon the line of sight visibility.

[0089] This embodiment also provides a second set of base stations 119 bwhich are provided in urban or built up areas. Each defines a“microcell” or “picocell” around it, to provide coverage in heavilyshadowed or built up urban areas. For example, within a building such asan airport or a train station, or along an underground railway, a numberof such picocell base stations 119 b are provided. Cover is thereforeprovided in areas where the base stations 119 a usually cannotcommunicate and satellites 4 will almost never communicate.

[0090] The base stations 119 include base station control circuits whichallocate frequencies for communicating with mobile terminals 112.

[0091] The transmit and receive antennas at the base stations 119 a aregenerally constrained to broadcast preferentially in the azimuthalplane, for example by using a suitable “apple core” torroidal or conicalreflector antenna, or are provided with some other beam shaping ordirecting means which reduces the gain above the azimuth (i.e. thehorizon) so that the beam shaping effectively mitigates the interferenceto the satellites; the transmit and receive antennas of the terrestrialhandsets 112 are generally omnidirectional to permit the handset to beused in any orientation.

[0092] Frequency Allocation

[0093]FIG. 6 shows the frequency allocations in this embodiment. Thefeeder link frequencies will not be discussed further in the followingembodiments, and the terms “satellite uplink” and “satellite downlink”hereafter will refer to the mobile links.

[0094] The satellite uplink bands may be within the 1985-2014 MHz rangeand the satellite downlink bans within the 2170-2200 MHz band range.These frequencies are generally referred to as S-band frequencies.

[0095] It will be seen that the satellite uplink frequency band occupiesspectrum not shared by the terrestrial network. Thus, the (relativelypowerful) transmission from the terrestrial base stations (andterminals) will not interfere with the (relatively weak) uplink signalsreceived at the satellite from the satellite mobile terminals.

[0096] Any non-urban base stations 119 a may also have allocated uplinkand downlink bands which do not interfere with the satellite uplink ordownlink bands. As is conventional in cellular mobile systems, thisspectrum is reused in geographically separate cells. In urban areas,there is a requirement for additional capacity since more users arepresent per square kilometer.

[0097] In this embodiment, the additional capacity is provided byreusing the satellite downlink, as shown in FIG. 6, to provideadditional terrestrial uplink and downlink bands. In this embodiment,the terrestrial uplink and downlink each occupy the same frequencies butare separated by a frequency space to permit frequency duplex separationwithin the base stations and the mobile terminals.

[0098] These are used by the base stations 119 b in microcells andpicocells; for the purpose of this embodiment, these are cells locatedinside buildings or tunnels. (For reasons discussed further below,additional frequencies are present within the satellite downlink bandwhich are not occupied by the additional terrestrial uplink and downlinkfrequencies.)

[0099] In such areas, the satellite downlink is frequently attenuated byceilings or walls. Since the power radiated by the satellite isrelatively low and the path length is relatively long, and the antennaused by a terrestrial user terminal has a relatively low gain (and/orG/T measure) as it is omnidirectional, the level of interference fromthe satellite into the terrestrial terminal is minimal.

[0100] Referring to FIG. 5, in this embodiment one or more of the basestations 119 a which are located in urban areas may also make use of theadditional frequency bands used by the pico base stations 119 b. This isbecause, as shown in FIG. 5, the level of shadowing by buildings makescommunication with satellites 4 a, 4 b difficult; only on the rareoccasions when a satellite 4 c is in an unobstructed line of sight to auser terminal will the user terminal be affected by the satellitedownlink.

[0101] Thus, to sum up, in this embodiment, the frequencies used by theterrestrial base stations 119 are allocated so that in urban or othershadowed areas, the additional terrestrial uplink and downlink bandslying within the satellite downlink band are utilised. The specificfrequency channels within the satellite downlink band are reused foruplink and downlink of the terrestrial basestations located in urban andsuburban areas.

[0102] In this embodiment, as the satellite communicates with each useron a narrow frequency channel, even if communication with a relativelysmall number of satellite users continues in the urban area, theinterference with the terrestrial uplink and downlink frequencies willbe minor because, due to the spectrum spreading of CDMA, theinterference on one particular frequency is (up to a certain level)absorbed in the CDMA error correction decoding. Thus, small numbers of(inherently low) satellite channels merely slightly raise the noisefloor.

[0103] Likewise, as the CDMA signals are spread over a wide spectrum,the noise power contributed by the PLMN 110 into any one of the narrowsatellite communication frequency channels is low.

[0104] In this embodiment, however, the effects of such residualinterference are reduced yet further by controlling the broadcast fromthe satellites 4. The orbits of each of the satellites 4 arecharacterised to a high degree of accuracy, and their inclinations areactively controlled to maintain their beam directions accuratelypointing to Earth. Each of the satellite spot beams has a radius of theorder of some tens or hundreds of kilometers. The spot beams of eachsatellite overlap, and those of one satellite overlap with those ofanother in the most regions of the Earth and at most times.

[0105] In this embodiment, the database station 15 maintains a databaserecording the positions of base stations 119 b using the additionalfrequency bands which lie within the satellite downlink. The frequenciesallocated to a given spot beam (which are dictated by a routing tableheld within each satellite and periodically reprogrammed from within thedata base station 15) are controlled so that the frequencies firstallocated (i.e. preferentially allocated when available) are those fromthe region of the satellite downlink not in use by terrestrial basestations 119. Thus, a number of satellite handsets may be operatedwithout any possibility of interference with the terrestrial network.

[0106] As the database station 15 communicates with the PLMNs 110periodically, it is able to vary the frequency allocations depending onthe instantaneous loads (i.e. demand for service) on the terrestrial andsatellite networks.

[0107] Frequencies are preferentially allocated from opposite ends ofthe shared frequency band; thus, for example, each time a new satellitecommunication channel is to be added, the next available frequency downfrom the high frequency end of the band may be allocated, whereas whereadditional terrestrial capacity is to be allocated, then frequencies maybe allocated from the next available frequency up from the low end ofthe band.

[0108] Where such frequencies are exhausted, the next to be allocated tocalls may be those which overlap with the terrestrial uplink band. Inthis embodiment it is easier to mitigate interference on the terrestrialuplink, since each base station 119 b can be provided with sophisticatedinterference reduction techniques to reduce the effect of suchinterference.

[0109] Finally, the last to be allocated are the frequencies sharing theterrestrial downlink frequency band in any spot beam which covers thearea overlying one of the base stations 119 b.

[0110] When the interference due to the above allocation exceeds acertain limit in such a way that this affects the capacity of theterrestrial network, dynamic reallocation of the uplink and downlinkfrequencies used by a number of terrestrial stations and the satellitenetwork is performed so that the overall interference is kept to aminimum. For example, the set of frequencies used by one spot beam ofthe satellite and its neighbours, and/or by one of the base stations andits neighbours, are varied to allocate non-interfering channels to thesatellite or the base station or both, exchanging those channels withthose of a neighbour.

[0111] The control channels of the satellite and/or the base station mayreallocate channels used on existing calls, by signalling to theterminals to hand over to a new frequency channel.

[0112] Thus, in the preferred arrangement of this embodiment, thesatellite downlink signals are selectively controlled to areas ofcoverage which include base stations 119 b which are reusing thesatellite downlink frequency, so as to mitigate the interference withthe terrestrial system.

[0113] Under circumstances where it is impossible to allocatenon-interfering spectrum to satellite users, it would be possible forthe satellite system to signal terminals to cease use of the satellitenetwork. It might be thought that the loss of satellite capacity wouldpreclude economic operation of a satellite system in this case. However,according to this embodiment it is envisaged that the vast majority ofsatellite user terminals 2 will be dual mode terminals as illustrated inFIG. 2. Accordingly, in areas where the satellite has shut down service,coverage through the terrestrial base stations 119 will be available.

SECOND EMBODIMENT

[0114] Referring to FIG. 7, in this embodiment, the satellite uplink isreused by the second set of base stations 119 b in urban areas as aterrestrial uplink, and the satellite downlink is reused by those basestations as a terrestrial downlink.

[0115] As in the preceding embodiment, the between the satellite systemand the terrestrial system is similarly small because of the blockingand shadowing effects of buildings.

[0116] In this embodiment, additional measures are taken to limit theinterference from the satellite user terminals into terrestrial basestations (satellite uplink into terrestrial uplinks), by providing thatthe satellite user terminals detect a signal indicating the possibilityof interference, and in response cease to transmit satellite signals onthe interfering channels and use non-interfering channels whereavailable.

[0117] Again, in this embodiment, it is envisaged that most of thesatellite user terminals are dual mode terminals. Referring to FIG. 8,in this embodiment, as distinct from the preceding embodiment, since thesatellite uplink and downlink spectra are the same as the additionalterrestrial uplink and downlink spectra, some of the radio frequencycomponents can be reused. FIG. 8 is based on FIG. 2b, and likecomponents are omitted from FIG. 8 for clarity.

[0118] In this embodiment, separate satellite and terrestrial antennas31 a, 31 b are maintained, since although the area of spectrum occupiedis the same, the satellite antenna preferably has a higher gain abovethe horizon whereas the terrestrial antenna will generally beomnidirectional.

[0119] A common RF amplifier block 52 comprising a low noise amplifier54 b on the downlink and power amplifier 54 a on the uplink is provided,connected switchably to either of the antennas 31 a, 31 b. The amplifiersection 52 is connected to a common up/down converter block 58consisting of an up converter converting from baseband to RF and a downconverter converting from RF to base band with a pair of switchablebandwidths corresponding to those of the satellite communicationschannels (which are relatively narrow) and terrestrial communicationschannels (which are relatively broad).

[0120] At base band frequency, the signal is then routed between theconverter block 58 and the separate codecs etc as discussed in relationto FIG. 2b. Thus, the expensive RF components need not be duplicated,resulting in reduced cost, weight and power consumption. Single modesatellite handsets 2 would omit the CDMA codec portion shown in FIG. 8.

[0121] In this embodiment, a special code indicating the frequencychannels used and the location of the terrestrial base station isdefined for transmission on the broadcast common control channel in eachspot beam. When a spot beam overlies a base station 119 b, which reusesthe satellite frequencies, the code is broadcast. When it is received byany satellite user terminal 2, the user terminal 2 responds by ceasingall uplink transmissions in shared channels by the satellite codec,until a control channel is detected in a satellite downlink spot beam onwhich the control signal is not being broadcast or the contents of thecontrol signal indicates a different frequency (indicating that the userterminal is now within coverage of a spot beam that does not overlap theterrestrial base stations 119 b).

[0122] The above described embodiment has the effect of causing allsatellite user terminals 2 which can receive the downlink on a beamwhich overlies one of the base stations 119 b to cease to generatesatellite signals on the shared frequency channels. However, firstly,since the beam may cover a wider area than the cell surrounding the basestation 119 b, many satellite user terminals 2 which could otherwisecommunicate with the satellite without interfering with the terrestrialbase station 119 are adversely affected. Secondly, satellite userterminals 2 which cannot receive the signal concerned (for examplebecause of fading or blockage) may nonetheless broadcast on thesatellite uplink channel and hence interfere with the terrestrial basestation 119.

[0123] To resolve the first of these problems, rather than sending abroadcast mode control signal which is to be acted upon by all satelliteuser terminals 2 within the beam, the position of each satellite userterminal 2 is registered with the data base station 15 (either byincorporating a GPS receiver within each user terminal which reports itsdata to the satellite periodically, or by using a range and Dopplerposition sensing technique as described above).

[0124] The data base station 15 compares the position of each to datadefining the coverage area of each of the base stations 119 b, and whena satellite user terminal 2 is detected to be within one of the coverageareas, a control signal of the type discussed above is transmittedspecifically to that satellite user terminal on a dedicated controlchannel therefore, or on a broadcast channel with a user terminaladdress decodable thereby. Thus, only those terminals which are detectedas being likely to interfere cease to be able to use the satellitesystem.

[0125] Alternatively, where the handsets are aware of their ownpositions (for example each is equipped with a GPS receiver), thecontrol signal may specify the co-ordinates of the coverage area of thebase station 119 b and each satellite user terminal 2 may be arranged toterminate uplink transmissions on shared frequency channels only if itlies within that coverage area.

[0126] To deal with the second problem identified above, in analternative embodiment, rather than making the satellite user terminalsresponsive to a control signal broadcast in the downlink from thesatellite to cease satellite mode transmissions, the terrestrial basestations 119 b which reuse the satellite uplink and downlink areequipped with a transmitter arranged to transmit the control signals.

[0127] The satellite codec within each satellite user terminal 2 whichcan receive transmissions from the base station 119 b (and hence mightgenerate uplink transmissions which would interfere with reception bythat base station) is arranged, on detecting the control signal, tocease transmissions by the satellite system codec on shared frequencychannels.

[0128] The control signal might simply be a beacon, broadcast at apredetermined frequency. Alternatively, it might emulate one of thesatellite broadcast control channels.

[0129] Thus, in this embodiment, with some small modification to thebase stations 119 b, only those satellite user terminals 2 which areactually within range of the base station 119 b are made unable tocommunicate with the satellite 4 using shared frequency channels, andthis is achieved regardless of whether the satellite downlink can bereceived by them or not. The terrestrial mobile codec of the dual modeterminal 2 in this embodiment (as in the last) does not requiremodification. This embodiment is effective not only with dual modesatellite terminals sets 2 but also with satellite user terminals whichlack a terrestrial mobile codec, because the broadcast signal from thebase station 119 b is received and acted upon by the satellite systemcodec.

[0130] Finally, rather than modifying the base stations 119 b, it ispossible instead to modify the terrestrial mobile codecs of dual modesatellite user terminals 2, so that such terminals continuously monitorthe downlink for signals from a terrestrial base station 119 b. Ondetection of a CDMA downlink signal, the terrestrial codec sends acontrol signal to the satellite system codec indicating the detectedterrestrial frequencies, to cause the satellite system codec to ceaseit's transmission on shared channels, and switch to free satellitechannels. On loss of signal from the base station 119, after apredetermined time without signal from the base station, the terrestrialsystem codec issues a control signal to the satellite system codecpermitting use once more on the shared channels when necessary.

[0131] This embodiment therefore has the advantage that minimalmodifications to the terrestrial base stations 119 are required.

[0132] It will be seen that this embodiment, in which the satelliteuplink spectrum is also available for terrestrial mobile uplink and thesatellite downlink spectrum is also available for terrestrial mobiledownlink, provides more bandwidth to the terrestrial network for sharingthan the preceding embodiment, and enables common RF components to beused in the satellite user terminal 2.

[0133] Unlike the preceding embodiment there is also the possibility ofinterference on the satellite uplink and the terrestrial uplink.

[0134] Since satellite uplink channels are on narrow frequency bands,the effect on the broadband terrestrial CDMA uplink channels of anyresidual satellite uplink transmissions is merely to increase slightlythe noise floor experienced.

THIRD EMBODIMENT

[0135] Referring to FIG. 9, in this embodiment, the frequency reuse ofthe previous embodiment is reversed. That is to say, the satelliteuplink is reused by the terrestrial downlink and vice versa. Thus,transmissions in the downlink from the satellite do not affect theterrestrial handsets, but could be received by the terrestrial basestations 119 b. Each such base station can, however, be protected fromtransmission from above by an overlying metal plate, or by suitablydesigning the antennas to reduce the gain and sidelobes in higherelevation angles and also by pointing the antennas tilted down from thehorizontal.

[0136] Thus, such shielding or beam shaping, in addition to theshadowing and blockage caused by the deployment of the base stations 119b indoors and in urban areas, substantially reduces the power levels onthe satellite downlink reaching the base stations 119 b.

[0137] For similar reasons, and because the antennas of the basestations 119 b are intended to broadcast predominantly in the azimuthalplane, the impact of the terrestrial downlink on the satellite uplink isminimal.

[0138] As in the preceding embodiments, the antennas of the basestations may either broadcast preferentially in the azimuthal plane orin all directions other than above the azimuth, so as to reduce thepower broadcast towards, and reduce the sensitivity to signals from, thesatellite 4.

[0139] Although the signals transmitted on the terrestrial and satelliteuplink by user terminals are of lower amplitude due to the lower poweravailable in the user terminals, it is noted that the terrestrial uplinksignals transmitted by terrestrial handsets could interfere with thesatellite downlink signals received by satellite mode handsets, and viceversa, where active terrestrial 112 and satellite mode 2 handsets areclose to each other.

[0140] Accordingly, in this embodiment, the techniques discussed in theabove first and second embodiments in reducing satellite transmissionson the satellite downlink and handset transmissions on the satelliteuplink are preferably employed.

[0141] Alternatively, each dual mode handset 2 of this embodiment may bearranged to detect CDMA transmission on the terrestrial uplink (i.e. theterrestrial uplink frequency used by other terrestrial handsets)throughthe satellite receiver . On detection of the frequency of transmissionsfrom a terrestrial handset 112, the satellite system codec is instructedto cease transmissions on any shared frequency channels on the satelliteuplink. Thus, where a dual mode terminal 2 is close enough to aterrestrial mode terminal 112 to detect transmission from it (and henceis likely to interfere with it) potentially interfering transmissionsfrom the dual mode handset 2 are terminated.

[0142] This embodiment has the advantage that the relatively powerfulsatellite and terrestrial downlink transmissions are received at thesatellite 4 and the base station 119, rather than at the user terminals2, 112, making interference at the user terminals less likely thaninterference at the satellite 4 and base station 119. Since it is easierto provide sophisticated interference mitigation and cancellationtechniques, of the type described in our earlier applications WO00/48333, WO 00/49735 or WO 00/35125 for example, at the network siderather than within the user terminals, the effects of any suchinterference can more easily be mitigated.

FOURTH EMBODIMENT

[0143] Referring to FIG. 10, in this embodiment, as in the first, theterrestrial uplink and downlink frequency bands both occupy one of thesatellite frequency bands. In this embodiment, however, it is thesatellite uplink frequency band which is shared. This can beaccomplished by placing a frequency gap between the terrestrial uplinkand downlink bands, allowing a frequency division duplexer to separatethe bands in the handsets and the base stations.

[0144] This embodiment is advantageous in situations where manysatellite user terminals 2 are connected to data terminal equipment suchas personal computers, personal digital assistants or other devices.Typically, such devices are used to download emails; or to downloadfiles via the Internet using either file transfer protocol (FTP) orhyper text transfer protocol (i.e. “web browsing”).

[0145] In such uses, the uplink needs to carry only occasional controland navigation commands specifying files to be downloaded, oracknowledging receipt of data, relative to the heavy usage of thesatellite downlink. There is, therefore, considerable scope for reusingthe satellite uplink.

[0146] As the data rate on satellite uplink channels will be low, theyare inherently more immune to the additional noise generated by the wideband CDMA PLMN traffic if each satellite uplink channel is allowed tooccupy the same bandwidth as the satellite downlink channel.Alternatively, the satellite uplink channels may be allocated a narrowerbandwidth, for example by time division multiplexing a higher number ofuplink channels together. The unused uplink channel frequencies thusreleased are available for terrestrial reuse.

Summary Of Interference Modes And Effects

[0147]FIG. 11 shows the satellite and terrestrial uplink and downlinks.TABLE 1 Interference Modes 1^(st) Embodiment 2^(nd) Embodiment 3^(rd)Embodiment 4^(th) Embodiment Satellite into POTENTIAL POTENTIALTerrestrial Base Satellite into POTENTIAL POTENTIAL Terrestrial UTSatellite UT into POTENTIAL POTENTIAL Terrestrial Base Satellite UTPOTENTIAL POTENTIAL into Terrestrial UT Terrestrial POTENTIAL POTENTIALBase into Satellite Terrestrial POTENTIAL POTENTIAL Base into SatelliteUT Terrestrial POTENTIAL POTENTIAL UT into Satellite TerrestrialPOTENTIAL POTENTIAL UT into Satellite UT

[0148] Referring to Table 1, the potential modes of interference in eachof the above embodiments are briefly discussed, together with thetechniques 5 preferred for mitigation thereof. It will be seen that inthe first embodiment there is potential interference from the satelliteinto the terrestrial base station 119 and user terminal 112; and fromthe terrestrial base station 119 and user terminal 112 into thesatellite user terminal 2. In the second embodiment there is potentialinterference from the satellite 4 into the terrestrial user terminal 112and vice versa, and from the base station 119 into the satellite userterminal 2 and vice versa.

[0149] In the third embodiment there is potential interference from thesatellite 4 into the terrestrial base station 119 and vice versa, andfrom the satellite user terminal 2 into the terrestrial user terminal112 and vice versa.

[0150] In the fourth embodiment there is potential interference in thesatellite user terminal 2 into the terrestrial base 119 and userterminal 112, and from the terrestrial base station 119 and userterminal 112 into the satellite 4.

[0151] Satellite 4 into Base Station 119

[0152] Using representative figures, providing a substantial number ofcontinuous satellite downlink channels (of the order of 40) wouldincrease the noise level, and hence reduce the effective cell sizeallowable for the terrestrial base station 119 by the order of 60%. Toreduce this impact on the PLMN 110, the following measures are proposed:

[0153] Providing suitable gain reduction above azimuth (for example byshielding, beam shaping or both) as discussed above can provide up to 25dB discrimination, halving the reduction in cell size.

[0154] As disclosed above, initial allocation by the database station 15of non-interfering channels to the satellite and terrestrial networksreduces the impact of the interference. Subsequently, shared frequencychannels are only allocated up to a predefined limit, which is decidedby the amount of interference. Finally, dynamically controlling thenumber of channels which overlap the PLMN bandwidth which can beallocated to satellite uplinks and downlinks depending on the relativeloading of the two networks, further assists in reducing the impact. Asmost or all satellite terminals 2 will be dual mode, they can alsooperate on the terrestrial PLMN 110 where needed.

[0155] In combination, these techniques greatly mitigate the impact ofinterference between the terrestrial and satellite systems.

[0156] Satellite 4 into Terrestrial Mobile Terminal 112

[0157] Using similar figures, it is estimated that up to 40 satellitedownlink channels could reduce the effective terrestrial cell size by ofthe order of 50% where these interfere with the terrestrial downlink.

[0158] To mitigate this, as above, the data base station 15 initiallypreferentially allocates channels to the satellite terminals 2 which donot overlap with the terrestrial spectrum, and dynamically controls thenumber of satellite channels sharing the spectrum where it is notpossible to avoid overlap. The combination of these techniqueseffectively mitigates potential interference.

[0159] Satellite User Terminal 2 into Terrestrial Base Station 119

[0160] The radio horizon experienced between the base station antennaand the user terminal will prevent interference from user terminals morethan, say, 30 km from the base station. However, within that distance,and to the extent not obstructed by obstacles, user terminals 2 caninterfere into the terrestrial base station 119 b where the uplinkspectra are shared.

[0161] To mitigate the interference, firstly, the satellite terminals 2are made dual mode, and are controlled as described above to operate asterrestrial mobile terminals and consequently to inhibit satelliteuplink transmissions while within the coverage of terrestrial basestations 119. This virtually eliminates the interference except where asatellite user terminal 2 is outside the coverage of a base station 119b but close enough to interfere with it, or where the satellite terminalis single mode only.

[0162] Secondly, as discussed above, the database station 15 takesadvantage of its knowledge of the locations of the base stations 119 andthe user terminals 2, to dynamically limit the uplink frequencyassignments for user terminals 2 close to a base station 119, tochannels which do not overlap the base station receive band. As themaximum interference range is estimated to be around 30 km, differentallocations can be made within different parts of each satellite beam.

[0163] The combination of these techniques virtually eliminates thepotential interference in this mode.

[0164] Satellite User terminal 2 into Terrestrial Mobile Terminal 112

[0165] The radio horizon between two handheld terminals is only around 8km, so that terminals further away from this will not interfere witheach other even where the uplink spectra are shared.

[0166] To mitigate this interference, the same techniques as in theprevious interference mode are operated, with the same results.

[0167] Terrestrial Base Station 119 into Satellite 4

[0168] As noted above, shadowing substantially reduces the direct lineof sight from urban base stations to the satellite. Further, asdiscussed above, the base station antennas are preferably designed tominimise gain at angles above the horizon, giving up to 25 dBdiscrimination in the direction of the satellites. Further, the databasestation 15 initially assigns satellite uplink channels which do notoverlap with the base station emission bandwidth for satellite beamswhich overlap terrestrial base stations 119 b; shared channels are onlyassigned as necessary.

[0169] Thirdly, as discussed above, dynamic control of the number ofshared channels allocated to satellite uplinks depending on relativeloading of the two networks is performed.

[0170] Terrestrial Base Station 119 into Satellite User Terminal 2

[0171] The interference situation is essentially the reverse of that forsatellite user terminal interference into the terrestrial base station,and the same techniques are used to mitigate interference.

[0172] Terrestrial Mobile Terminal 112 into Satellite 4

[0173] To mitigate this interference, the data base station 15 initiallyassigns satellite uplink channels which do not overlap with the basestation downlink bandwidth in areas where satellite beams overlap cellsof base stations 119 b; shared channels are only assigned as thesatellite or terrestrial systems reach capacity. Secondly, the number ofchannels shared is dynamically controlled in dependence on the relativeloading of the two networks and thereby the interference between boththe networks are minimised.

[0174] Terrestrial Mobile Terminal 112 into Satellite User Terminal 2

[0175] As above, the maximum interference range between the two userterminals is only 8 km. To mitigate interference, firstly, as above, thefact that the satellite terminals 2 are dual mode causes them to operatewhenever within range of a base station 119 b as terrestrial mobileterminals, which eliminates most of interference except where theterrestrial terminal 112 is in communication with a base station 119 band is within range of a satellite user terminal 2 which is blocked orotherwise prevented from communicating with the terrestrial network 119b.

[0176] Secondly, the data base station 15 in conjunction with the MSC116 dynamically allocates the satellite and terrestrial uplink anddownlink frequency assignments for satellite user terminals near a basestation 119 to channels that do not overlap each other.

OTHER EMBODIMENTS

[0177] It will be clear from the foregoing that the above describedembodiments are merely a few ways of putting the invention into effect.Many other alternatives will be apparent to the skilled person and arewithin the scope of the present invention.

[0178] For example, although the above-described embodiments mentionbase stations sited indoors or in urban areas, and thus make use of thepotential shadowing there, it will be clear that the variousinterference mitigation techniques and spectrum reuse techniquesdescribed could also be used with base stations sited additionally oralternatively in suburban or rural areas.

[0179] Although in the above embodiments a limit is set on the number ofinterfering frequencies used, a limit based on other criteria such asthe total interfering power (calculated for example taking into accountpath loss and power used on each channel, and/or other criteria), mayinstead be used.

[0180] Whilst in certain of the above embodiments, frequency duplexingis used to share satellite spectrum between the terrestrial uplink anddownlink, time division duplex between the terrestrial uplink anddownlink could alternatively be used (as in certain existing terrestrialnetworks).

[0181] It will be clear that other possibilities for reuse byterrestrial networks of the satellite spectrum exist, making use of theabove described observations and techniques. Further, the abovedescribed techniques may be combined.

[0182] Further, it will be clear that each of the above describedtechniques for reducing the interference between the satellite andterrestrial systems, or for detection techniques to do so, may beemployed separately of the others, in other similar interferencescenarios.

[0183] Whereas in the above described embodiments a dual mode userterminal comprises a common housing and user interface containingseparate satellite system and terrestrial codecs, other constructionsare possible; for example it could comprise separate single modeterminals interconnected by a wire or a wireless interface.

[0184] The CDMA can be third generation wide band CDMA (W-CDMA) or CDMA2000.

[0185] Whereas a TDMA/FDMA satellite system and CDMA terrestrial systemare described above, in principle the satellite system could be CDMA andthe terrestrial system TDMA/FDMA or FDMA.

[0186] The numbers of satellites and satellite orbits indicated arepurely exemplary. Smaller numbers of geostationary satellites (forregional coverage), or satellites in higher altitude orbits, could beused; or larger numbers of low earth orbit (LEO) satellites could beused, as disclosed in EP 0365885, or publications relating to theIridium or Teledesic systems, for example. Equally, different numbers ofsatellites in intermediate orbits could be used. In principle, evenflying platforms such as balloons or aircraft are not excluded.

[0187] It will be understood that components of embodiments of theinvention may be located in different jurisdictions or in space. For theavoidance of doubt, the scope of the protection of the following claimsextends to any part of a telecommunications apparatus or system or anymethod performed by such a part, which contributes to the performance ofthe inventive concept.

We claim:
 1. A communications system comprising a satellite mobilecommunications network which comprises a plurality of satellites and aplurality of user terminals communicating on satellite uplink anddownlink bands; and a terrestrial mobile communications network whichcomprises a plurality of base stations and a plurality of user terminalscommunicating on terrestrial uplink and downlink bands; characterised inthat at least one of the terrestrial bands at least partly reuses atleast one of the satellite bands.
 2. A system according to claim 1, inwhich said base stations comprise second base stations reusing saidsatellite bands, said second base stations being provided only in areaswhere the path from said satellites to the user terminals will beshadowed.
 3. A system according to claim 2, in which said areas areenclosed spaces.
 4. A system according to claim 2, in which said areasare urban areas.
 5. A system according to claim 1, in which saidsatellite mobile communications network communicates infrequency-divided fashion, using relatively narrow frequency channelswithin said bands.
 6. A system according to claim 1, in which saidterrestrial mobile communications network communicates in code-dividedfashion, using relatively wide frequency channels within said bands. 7.A system according to claim 1, in which said terrestrial uplink anddownlink bands at least partly reuse said satellite downlink band.
 8. Asystem according to claim 7, in which said terrestrial bands do notreuse said satellite uplink band.
 9. A system according to claim 1, inwhich said terrestrial uplink and downlink bands at least partly reusesaid satellite uplink band.
 10. A system according to claim 9, in whichsaid terrestrial bands do not reuse said satellite downlink band.
 11. Asystem according to claim 1, in which said terrestrial uplink bandreuses said satellite uplink band, and said terrestrial downlink bandreuses said satellite downlink band.
 12. A system according to claim 1,in which said terrestrial downlink band reuses said satellite uplinkband, and said terrestrial uplink band reuses said satellite downlinkband.
 13. A system according to claim 1, further comprising a channelallocator allocating channels to be used by at least one of saidnetworks, in dependence upon the frequencies allocated to the other. 14.A system according to claim 13, in which the channel allocator isarranged to control the frequencies allocated to both said networks. 15.A system according to claim 13, in which the channel allocator isarranged to allocate a channel for use by a terminal to communicate withone of said networks initially from a set of frequencies not used by theother said network in the region of the terminal, where such anon-interfering frequency is available.
 16. A system according to claim13, in which the channel allocator is arranged to allocate a channel foruse by a terminal to communicate with one of said networks from a set offrequencies also used by the other said network in the region of theterminal, provided that less than a predetermined measure ofinterference is thereby reached.
 17. A system according to claim 16, inwhich said level is determined by a number of said channels.
 18. Asystem according to claim 16, in which, when said level is reached, thechannel allocator is arranged to use frequency planning and terminal andnetwork location information to dynamically allocate shared frequencychannels.
 19. A dual mode user terminal for use in a system according toany claim
 1. 20. A terminal according to claim 19, in which there isprovided a common radio frequency circuit shared by a satellite systemcontrol circuit and a terrestrial system control circuit.
 21. A terminalaccording to claim 19, arranged to cease usage of frequencies sharedbetween the satellite and terrestrial systems on detection ofpredetermined conditions associated with the proximity of saidterrestrial mobile communications network, to prevent interferencetherewith.
 22. A terminal according to claim 21, in which thepredetermined conditions comprise detection of a control signaltransmitted by a said satellite.
 23. A terminal according to claim 21,in which the predetermined conditions comprise detection of a signaltransmitted by a said base station.
 24. A terminal according to claim21, in which the predetermined conditions comprise detection of a signaltransmitted by a user terminal in the terrestrial uplink band.
 25. Asatellite communications network for use in the system of claim
 1. 26. Anetwork according to claim 25, comprising a control station arranged toreduce use of said satellite downlink and/or uplink in regions aroundone of said base stations.
 27. A network according to claim 25,comprising a control device arranged to transmit a control signal tosatellite user terminals in regions around one of said base stations tocause said user terminals to reduce use of said satellite uplink.
 28. Aterrestrial communications network for use in the system of claim
 1. 29.A network according to claim 28, comprising a control device arranged totransmit a control signal to satellite user terminals in regions aroundone of said base stations to cause said user terminals to reduce use ofsaid satellite uplink.
 30. A method of allocating communicationsspectrum to base stations of a terrestrial mobile communicationsnetwork, in which a frequency band interferes with channels of asatellite communications system, comprising allocating said frequencyband preferentially to base stations in areas where shadowing willreduce the level of communications with the satellites of said satellitecommunications system.
 31. A method of reusing frequency bands betweenbase stations of a terrestrial mobile communications network and asatellite communications network, comprising allocating said frequencybands using integrated resource management and other mitigationtechniques in a way to minimise interference between both the systemsand thus making optimum usage of valuable frequency spectrum.